The present invention relates to a Stirling refrigerator which can cool e.g. an infrared sensor at temperatures as cryogenic as e.g. 80 K.
Referring now to FIG. 5, there is shown the structure of a conventional Stirling refrigerator.
In FIG. 5, the Stirling refrigerator is mainly constituted by a compressor 1, a cold finger 2 and a transfer pipe 3 connecting the compressor 1 and the cold finger 2. The compressor 1 includes a first cylinder 4a, a second cylinder 4b, a first piston 5a and a second piston 5b. Locating the first piston 5a and the second piston 5b is obtained by supporting springs 6a and 6b. The compressor has such a structure that the first piston 5a and the second piston 5b reciprocate in the first cylinder 4a and the second cylinder 4b, respectively.
To the first piston 5a and the second piston 5b are coupled a first sleeve 7a and a second sleeve 7b, respectively, which are made of non-magnetic, light-weight material. On the sleeves 7a and 7b are wound electric conductors, respectively, to form a first movable coil 8a and a second movable coil 8b. The movable coils 8a and 8b are connected to first lead wires 10a and 10b, and second lead wires 11a and 11b which extend outside through the wall of a housing 9. The lead wires 10a, 10b, 11a and 11b have first electric contacts 12a and 12b, and second electric contacts 13a and 13b which are outside of the housing 9. In the housing 9 are provided permanent magnets 14a and 14b, and yokes 15a and 15b which form closed magnetic circuits, respectively. The compressor has such a structure that the movable coils 8a and 8b can reciprocate in the axial election of the pistons 5a and 5b in a first gap 16a and a second gap 16b, respectively, the first gap 16a and the second gap 16b being formed in the closed magnetic circuits comprising he permanent magnets 14a and 14b, and the yokes 15a and 15b, respectively. In the gaps 16a and 16b are produced permanent magnetic fields in radius directions transverse to the moving direction of he movable coils 8a and 8b.
The internal space which is defined by the cylinders 4a and 4b, and the pistons 5a and 5b is called a compression space 17. The compression space 17 has working gas such as helium gas sealed in it under a higher pressure. In order to prevent the working gas in the compression space 17 from leaking through the gap between the cylinder 4a and the piston 5a, and through the gap between the cylinder 4b and the piston 5b, seals 28a and 28b are arranged in these gaps. This is the structure of the compressor 1.
On the other hand, the cold finger 2 includes a cylindrical cold cylinder 18, and a displacer 20 which is engaged with a resonant spring 19 and can slidably reciprocate in the cold cylinder 18. The internal space of the cold cylinder 18 is divided into two parts by the displacer 20. The upper space above the displacer 20 is called a cold space 21, and the lower space under the displacer 20 is called a hot space 22. In the displacer 20 are arranged a regenerator 23 and a gas passage hole 24. The cold space 21 and the hot space 22 are interconnected through the regenerator 23 and the gas passage hole 24. The regenerator 23 is filled with are generator matrix 25 such as a plurality copper wire mesh screens. In order to prevent a working gas from leaking through the gap between the cold cylinder 18 and the displacer 20, a seal 26 is arranged in the gap between the displacer 20 and the cold cylinder 18. The spaces of the cold finger 2 have the working gas such as helium gas sealed therein under a high pressure like the compressor 1. This is the structure of the cold finger 2. The compression space 17 of the compressor 1 is interconnected to the hot space 22 of the cold finger 2 though the transfer pipe 3. The compression space 17, the internal space in the transfer pipe 3, the cold space 21, the hot space 22, the regenerator 23 and the gas passage hole 24 are connected in series. They are called a working space 27 as a whole.
The operation of the conventional refrigerator thus constructed will be explained.
When an a.c. current is applied to the movable coils 8a and 8b through the electric contacts 12a, 12b, 13a and 13b, and the lead wires 10a, 10b, 11a and 11b, the movable coils 8a and 8b are subjected to a Lorentz force in the axial direction due to interaction of the magnetic fields in the gaps 16a and 16b, respectively. As a result, assemblies constituted by the pistons 5a and 5b, the sleeves 7a and 7b, and the movable coils 8a and 8b move horizontally in the axial direction of the pistons, respectively.
Suppose that the first movable coil 8a and the second movable coil 8b have the same properties, and that the strength of the magnetic field in the first gap 16a and that in the second gap 16b are equal to each other. When a sinusoidal current is applied to the first movable coil 8a and the second movable coil 8b to make them vibrate with the same amplitude in opposite directions, the pistons 5a and 5b reciprocate in the cylinders 4a and 4b in the opposite directions, giving sinusoidal undulation to the gas pressure in the working space 27 which extends from the compression space 17 to the cold space 21.
Changes in the flow rate of the gas passing through the displacer 20 and the regenerator 23 due to such sinusoidal undulation cause the displacer 20 including the regenerator 23 to axially reciprocate in the cold finger 2 at the same frequency as and out of phase with the pistons 5a and 5b.
When the pistons 5a and 5b, and the displacer 20 are moving keeping a suitable difference in phase, the working gas sealed in the working space 27 performs a thermodynamic cycle known as the "Inverse Stirling Cycle", and generates cold production mainly in the cold space 21. The "Stirling Cycle" and the principle of generation of the cold production thereby are described in detail in "Cryocoolers" (G. Walker, Plenum Press, New York, 1983, PP 117-123). The principle will be described briefly. The working gas in the compression space 17 which has been compressed by the pistons 5a and 5b to be heated is cooled while flowing through the transfer pipe 3. The gas thus cooled flows into the hot space 22, the regenerator 23 and the gas passage hole 24. The gas is precooled in the regenerator 23 by the cold production which has been accumulated in a preceding half cycle, and then enters the cold space 21. When most of the working gas has entered the cold space 21, expansion starts to generate cold production in the cold space 21. After that, the working gas returns through the same route in the reverse order, passing the cold production to the regenerator 23, and enters the compression chamber 17. At that time, heat is removed from the leading portion of the cold finger 2, causing the surroundings outside the leading portion to be cooled. When most of the working gas has returned to the compression chamber 17, compression restarts, and the next cycle commences. The process as described above is repeated to complete the "Inverse Stirling Cycle", causing cold production to generate.
The conventional refrigerator involves the problem as described below. Because locating the assemblies constituted by the pistons, the movable coils and the sleeves is obtained by the supporting springs, the respective assemblies constitute a spring-mass vibration system having one degree of freedom. Referring now to FIG. 6, there is shown a model diagram of the spring-mass vibration system. In FIG. 6, symbol m designates the mass of each assembly which comprises the piston, the movable coil and the sleeve. Symbol k designates the spring constant of each supporting spring. Symbol f.sub.O designates the resonance frequency of the vibration system. The symbol f.sub.O is defined by the following equation, using the symbols m and k: ##EQU1## Suppose that the conventional refrigerator is arranged at such environment that it is subjected to vibration from outside in the case of e.g. an air craft or a vehicle. When vibration having a component of f=f.sub.O in the axial direction of the pistons is applied to the conventional refrigerator, the assemblies which comprise the pistons, the movable coils and the sleeves resonate, so that the assembly of the first piston, the first movable coil and the first sleeve, the assembly of the second piston, the second movable coil and the second sleeve, and the working gas in the compression space vibrate with the same cycle and with the same phase as one unit as shown in FIG. 6. Since no vibration damping effect due to the working gas in the compression space is involved in such resonance, resonant magnification is great to obtain vibration having wide amplitude.
This creates a problem in that when the vibration which is applied from outside grows greater, the vibrating pistons, the vibrating movable coils and the vibrating sleeves can collide against the housing or the yokes to make a noise or damage a part.
It is an object of the present invention to resolve the problem and to provide a cryogenic refrigerator capable of damping resonance without making a noise and damaging a part.